In the coming year we plan to expand our studies on substances other than divalent cations which may affect PTH secretion. In particular we will continue investigations of the effects of vitamin D and its various metabolites in particular, 24,25(OH)2D3, on hormone secretion; studies which suggest a physiologic feedback control between the major calcium regulating hormones. For these studies we will utilize the uremic canine model of hyperparathyroidism. In addition, we will study the effects of gastrointestinal hormones and inhibitors of hormones on PTH secretion. Our long term studies of the mechanisms involved in the genesis and development of renal osteodystrophy will be continued. We are presently studying the control of phosphate excretion in this disease state. We will investigate the effects of altering the calcium status of the animals for the effect it may have on vitamin D's ability to effect PTH secretion.